Enhance server start command with language support validation (#6)

* Initial plan

* Add lsp server test command and tests

Co-authored-by: observerw <20661574+observerw@users.noreply.github.com>

* Fix linting issues in server test command

Co-authored-by: observerw <20661574+observerw@users.noreply.github.com>

* Address code review feedback - use BaseLSPTest and dynamic language list

Co-authored-by: observerw <20661574+observerw@users.noreply.github.com>

* Use pytest tmp_path fixture for cleaner test implementation

Co-authored-by: observerw <20661574+observerw@users.noreply.github.com>

* Merge test command features into server start command

- Remove separate 'lsp server test' command
- Enhance 'lsp server start' with better error messages
- Add path existence check before attempting to start server
- Show friendly error with supported languages list when language not supported
- Remove tests for standalone test command

Co-authored-by: observerw <20661574+observerw@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: observerw <20661574+observerw@users.noreply.github.com>

Author: Copilot

src/lsp_cli/server.py

@@ -1,7 +1,9 @@
 from pathlib import Path
 
 import typer
+from lsp_client.clients.lang import lang_clients
 
+from lsp_cli.client import find_client
 from lsp_cli.manager import (
     CreateClientRequest,
     CreateClientResponse,
@@ -56,6 +58,36 @@ def start_server(
     ),
 ):
     """Start a background LSP server for the project containing the specified path."""
+    # Check if the path exists
+    if not path.exists():
+        print(f"Error: Path does not exist: {path.absolute()}")
+        raise typer.Exit(1)
+
+    # Try to find a language client for this path
+    target = find_client(path)
+
+    if target is None:
+        # No language support found - generate list of supported languages dynamically
+        supported_langs = sorted(
+            {
+                client_cls.get_language_config().kind.value
+                for client_cls in lang_clients.values()
+            }
+        )
+        supported_langs_str = ", ".join(supported_langs)
+
+        print(f"Error: Language not supported for path: {path.absolute()}")
+        print()
+        print("The CLI cannot analyze code files in this language.")
+        print(f"Supported languages: {supported_langs_str}")
+        print()
+        print("Please check:")
+        print("  - The file extension matches a supported language")
+        print(
+            "  - The project has the required language markers (e.g., go.mod, Cargo.toml)"
+        )
+        raise typer.Exit(1)
+
     with get_manager_client() as client:
         resp = client.post(
             "/create", CreateClientResponse, json=CreateClientRequest(path=path)